The present invention relates to a method of machining a workpiece by electric discharge while holding the workpiece on a mounting stand in an electric discharge machining process consisting of a series of machining steps. The invention also relates to an apparatus for practicing the method.
In an electric discharge machining operation with an electric discharge machining apparatus such as a wire electric type electric discharge machining apparatus or an engraving type electric discharge machining apparatus, it is generally required to change electric discharge machining conditions in each of a plurality of machining steps, namely, in each of coarse, medium and finish machining steps, which are generally carried out in the stated order.
In a conventional electric discharge machining apparatus, as disclosed by Japanese Published Patent Application No. 3079/1972, in each of the machining steps mentioned above, the respective machining conditions, such as discharge current, discharge voltage, pulse width, and pause time, are set either according to a standard machining condition table or manually according to the experience and intuition of the operator.
In an NC (numerically controlled) electric discharge machining apparatus developed recently, machining conditions and machining evaluation data such as machining speed, electrode consumption, and machined surface roughness in combination are complied empirically based on a standard machining operation and are stored as data files in memory. Access is made to the data files as required during subsequent machining operations.
The conventional electric discharge machining apparatus designed as described above is disadvantageous in that the evaluation of the actual machined surface roughness for set machining conditions depends entirely on the operator; that is, the evaluation is not always correct. In addition, it is very frequently necessary to suspend the machining operation to evaluate the machined surface roughness. Thus, the total machining time is long and it is not possible to automate the machining operation.
On the other hand, in the NC apparatus, the data files are the accumulation of data for standard machining conditions, and therefore it is impossible to set detailed machining conditions as the actual machining operation progresses. For instance, in the case where, during a machining operation it is required to change from first machining conditions to second machining conditions by switching data files, the second machining conditions must be made available sufficiently before the position where the conditions are to be switched. That is, since the machined surface roughness is not monitored during the machining operation and the machining operation is carried out on a continuous basis, it is always necessary to carry out each stage of machining for a sufficiently long period as to guarantee that the desired surface roughness for that stage is attained. Therefore, in practice, much machining time is wasted since machining will often continue when the desired surface roughness has already been obtained.